In conventional belt tensioner arrangements, the belt load force applied during installation, while the mounting bolt is still loose, has a natural tendency to apply a tilting force to the tensioner. More specifically, the loose bolt tends to pivot or tilt around the area at which the bolt enters into mounting structure. The installation shaft and pivot shaft also tend to bend or pivot about this area as a result of the belt load force and loose mounting bolt.
When the bolt is tightened after the installation shaft is rotated to move the tensioner to the nominal installation position, the bolt, installation shaft, and pivot shaft tend to straighten against the tilting action of the belt load force. It can be appreciated that this straightening action can cause the tensioner arm to rotate away from its nominal installation position. As a result, the tensioner might not be installed in its proper static position. While a position indicating pointer connected with the belt tensioner lever arm might be brought out of alignment with a position correct indicator during this tightening of the bolt to alert the operator to re-adjust the tensioner, the inaccurate positioning of the tensioner may go unnoticed due to the fact that the hysteresis forces (friction) of the tensioner may be restricting the final rotation of the belt tensioner lever arm when the tensioner and belt are static. The incorrect tensioner location may, therefore, only be detected after first running the engine (which will release the frictional "stickiness" of the tensioner).
In order to reduce the tilting of the tensioner during the installation process and the consequent unwanted movement of the lever arm caused by the final tightening of the mounting bolt, it has also become customary to specify a precise pre-tightening torque for the mounting bolt prior to the application of the installation force. The drawbacks of this practice are twofold. First, it increases the frictional forces between the tensioner and the tensioner mounting surface on the engine, requiring higher installation forces. This may also cause possible scarring of the surfaces resulting in tensioner angularity errors. Second, the practice adds extra labor and cost on the engine assembly line without satisfactorily solving the problem of erratic tensioner positioning.
U.S. Pat. No. 5,244,438 ('438 patent) discloses a belt tensioner that employs a flanged eccentric bush that extends in an axial direction essentially abutting the belt centerline plane. There are a number of shortcomings to this design. First, the flanged eccentric bush is truncated in that its axial extent terminates at the same location in which its engagement with the surrounding bearing neck terminates. This truncated configuration is impractical from a manufacturing and assembly standpoint. For example, because the eccentric bush is truncated, it may fall out from the bearing neck under the force of gravity (perpendicular to the bush axis when mounted on an engine) prior to the mounting screw being employed to mount the tensioner on the engine.
U.S. Pat. No. 5,759,125 suggests a belt tensioner arrangement comprising a rigid support base having a longitudinal slot for allowing translational displacement of the support base to adjustably secure the support base to the vehicle engine. This movement is not restricted to an arcuate, connecting-rod type movement, but is free to move in any direction as dictated by an externally applied force. This freedom of movement is no better or even worse than the connecting rod type arcuate movement with regard to proper alignment of the tensioner during installation.